People are paying greater attention to the environmental impact of everything they do, with businesses, governments and individuals becoming increasingly aware of the need for sustainability and subsequently this is being reflected in their actions. Chemists are no different, and play a key role in the movement towards a sustainable future, applying their trade in the search for innovative green solutions across a myriad of industries. However, chemists themselves produce a large amount of waste through their extensive use of organic solvents, as media in which to carry out reactions.
Below we look into the waste caused by organic solvents, and how water can be a sustainable alternative.
Organic solvents are typically hazardous both to safety and health; they are often flammable (e.g. acetone) and have skin irritant properties, but may also be carcinogenic (e.g. benzene), have reproductive hazards (e.g. 2-ethoxyethanol), or may be neurotoxins (e.g.n-hexane). These characteristics mean they should never be discharged into the environment and instead should be collected and incinerated, at which point they contribute to greenhouse gas emissions. These chemicals are also often sourced from fossil fuel reserves: finite resources utilised for countless purposes globally - this itself doesn’t hark “sustainable”.
The search for substitute media in which to carry out these reactions, including the use of supercritical CO₂, ionic liquids and biomass-derived organics, is progressing, but each has limitations. Another option that has garnered the attention of many chemists over recent years is actually water - considered “nature’s solvent”¹, water is abundant, non-toxic, non-flammable and cheap. The physical properties of water, and how they change with temperature and pressure, allow many more reactions than are first expected to take place within this polar solvent. Water has a large dielectric constant and high polarity, which leads to the hydrophobic effect - the clustering of non-polar molecules or functionalities to reduce the polar/non-polar interfacial area. This can concentrate pockets of non-polar reactants, influencing their reaction rate, chemo- and regioselectivity². Reactions that take place in conditions like this are termed ‘on-water’, and the most well-documented example is the acceleration of the Diels-Alder reaction with water as solvent, as shown by Rideout and Breslow, 1980³. The ability to modulate the hydrophobic effect of water with the addition of different salts in salting-in and salting-out was further shown in this work. Volatile organic compounds (VOCs) have traditionally been used as solvent media for reactions such as this.
The heating and pressurisation of water to subcritical or supercritical states provides further options in the use of water as a solvent, though brings safety concerns and energetic requirements that may limit the desirability and application of these conditions. These changes in state drastically change the properties of water, to have a low dielectric constant, low polarity and increased dissociation. Supercritical water is therefore a highly suitable solvent for non-polar compounds, allowing their reaction throughout the bulk reaction mixture. Further, the increased dissociation of H+ and OH- ions provide both acidic and basic catalysis, without needing to alter pH.
Another promising option for reaction using water as a solvent is the use of surfactants to provide better interaction between polar and nonpolar molecules in ‘in water’ reactions⁴. The Lipshutz group at UC Santa Barbara focus on improving green chemistry in synthetic organic chemistry, and one of their focuses are “designer surfactants” that can, when present in water in very limited quantities, provide nanometer micelles or so-called “nanoreactors” for non-polar reactions to take place. Lipshutz and his group have been able to undertake many big-name organic reactions using water as the solvent, including Suzuki-Miyaura, Sonogashira, Mizoroki-Heck and Negishi couplings⁽⁵⁾ and Lipshutz states: “Micellar catalysis is becoming rich with a growing toolbox of technologies that enable just about any reaction to be run in water.”⁽⁶⁾
The limited quantities of designer surfactants needed has the above-described benefit of “higher local substrate concentrations and hence, faster reaction rates” often also reducing the catalyst loadings and energy required for reactions⁴. Relatively mild conditions are actually often necessitated by the need to avoid surfactants reaching their cloud-point, where the properties of surfactant solutions change. Further, the work-up required for isolation and purification of reaction products can be majorly simplified by using in-water reaction conditions, with work up including ““in flask” extraction of the product from the aqueous reaction mixture using a single, minimal amount of a recyclable organic solvent, or by simply decanting or filtering to obtain the solid product”⁴. Other benefits of in-water reaction conditions include the ability to perform sequential reaction steps in one pot without work-up and particularly to merge sequential reactions using chemocatalysis and biocatalysis (using enzymes that may be deactivated by organic solvents)⁽⁷⁾.
Water appears to hold immense promise as a green solvent for the chemical industry and the environment. It is inevitable that changes will be made to current chemical practices, whether due to the reducing supply or more difficult sourcing of organic solvents or for economic as well as environmental reasons. Prioritising the development of water-based technologies would bring major benefits.
ELGA LabWater is established in the supply of high purity water for application in chemistry and related practices, with ELGA purifiers reliably and consistently providing high purity water that can be used with confidence to support the aforementioned ecologically driven changes in traditional Organic Chemistry. The environmental focus of the parent Veolia group makes ELGA the sustainable choice for high purity water in the laboratory.
Dr Bethany Campbell
After completing a BA and MSc in Natural Sciences at the University of Cambridge, Bethany was awarded a PhD in Chemical and Process Engineering from the University of Surrey. During her PhD, Bethany investigated the treatment of process water from the hydrothermal carbonisation of biomass - a process for the valorisation of waste to produce hydrochar (a fossil coal alternative). Bethany is now the R&D Wastewater Specialist at ELGA, where she works to develop technologies and products for treatment of wastewaters.